Purchase order Subitem Category

Hi!

I have a scenario where when we purchase part A from the vendor we must always purchase either parts B and C.  All three of the parts have there own standard costs and parts B and C can be purchased individually.

So when we issue the purchase order I need part B or C to be linked to part A as a lower level item. I also need a standard to appear to both part numbers as we need to receive both in SAP.

I explored the use of discount in kind but I can't have part B/C to appear as zero dollars otherwise I'm going to run into purchase price variances.

Any ideas?

Thanks in advance Guru's!